The present invention relates to a focus device for a zoom lenses, having coverage in a range from an ordinary photographing lens to that of close-up photographing.
Recently, there has been provided a so-called macro zoom lens or close focussing zoom lens which is employed for close-up photographing over a range beyond that of ordinary photographing. Many methods therefor have been practically used.
The first method is that an additional close-up lens is disposed at either end of the zoom lens system or in the middle of the zoom lens system. The second method is that a part of a variating lens system of the zoom lens system is moved. The third method is that a focussing lens of the zoom lens system is moved. The fourth method is that the overall zoom lens system is moved.
The first method has the following defects.
1. Because of the addition of the new optical system, the number of lens elements in the construction of the zoom lens is increased to thereby deteriorate the transmission factors such as inner reflection and the like.
2. When a close-up lens is used as a lens attachment, an operation to mount it to a lens mount barrel of the zoom lens is required, hence the operationability is deteriorated. Also, since the close-up lens is a separate component there is a possibility that it will be lost.
3. In case where the close-up lens is mounted inside of the zoom lens mount barrel, during the ordinary photographing, it is necessary that the close-up lens be displaced or offset from the optical path. For this reason, the size of the zoom lens barrel is enlarged.
4. A close-up or short distance scale must be added on an ordinary distance scale so the scale display is more complicated.
The second method has the following defects.
1. The moving amount of a part of the variator lens system is varied according to the focal distance. It is, therefore, very difficult to display the short distance scale or the construction of the scale becomes rather complicated.
2. Ordinary operational members for focussing and operational members for short distance focussing are independent elements. Accordingly, the handling or operation thereof is complicated and at the same time, means for distinguishing the variator operation from the short distance focussing operation is required.
The third and fourth methods have the following defects.
1. A focus lens system which is not generally movable is moved during short distance photographing. Therefore, the reproducibility of the positional relation thereof deteriorates.
2. Since the advancing amount is varied according to the focal length, the scale display is complicated.